.\"	$OpenBSD: SSL_CTX_set_cert_verify_callback.3,v 1.1 2016/11/05 15:32:19 schwarze Exp $
.\"	OpenSSL b97fdb57 Nov 11 09:33:09 2016 +0100
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.Dd $Mdocdate: November 5 2016 $
.Dt SSL_CTX_SET_CERT_VERIFY_CALLBACK 3
.Os
.Sh NAME
.Nm SSL_CTX_set_cert_verify_callback
.Nd set peer certificate verification procedure
.Sh SYNOPSIS
.In openssl/ssl.h
.Ft void
.Fo SSL_CTX_set_cert_verify_callback
.Fa "SSL_CTX *ctx"
.Fa "int (*callback)(X509_STORE_CTX *, void *)"
.Fa "void *arg"
.Fc
.Sh DESCRIPTION
.Fn SSL_CTX_set_cert_verify_callback
sets the verification callback function for
.Fa ctx .
.Vt SSL
objects that are created from
.Fa ctx
inherit the setting valid at the time when
.Xr SSL_new 3
is called.
.Sh NOTES
Whenever a certificate is verified during a SSL/TLS handshake,
a verification function is called.
If the application does not explicitly specify a verification callback
function, the built-in verification function is used.
If a verification callback
.Fa callback
is specified via
.Fn SSL_CTX_set_cert_verify_callback ,
the supplied callback function is called instead.
By setting
.Fa callback
to
.Dv NULL ,
the default behaviour is restored.
.Pp
When the verification must be performed,
.Fa callback
will be called with the arguments
.Fn callback "X509_STORE_CTX *x509_store_ctx" "void *arg" .
The argument
.Fa arg
is specified by the application when setting
.Fa callback .
.Pp
.Fa callback
should return 1 to indicate verification success and 0 to indicate verification
failure.
If
.Dv SSL_VERIFY_PEER
is set and
.Fa callback
returns 0, the handshake will fail.
As the verification procedure may allow the connection to continue in case of
failure (by always returning 1) the verification result must be set in any case
using the
.Fa error
member of
.Fa x509_store_ctx
so that the calling application will be informed about the detailed result of
the verification procedure!
.Pp
Within
.Fa x509_store_ctx ,
.Fa callback
has access to the
.Fa verify_callback
function set using
.Xr SSL_CTX_set_verify 3 .
.Sh SEE ALSO
.Xr ssl 3 ,
.Xr SSL_CTX_load_verify_locations 3 ,
.Xr SSL_CTX_set_verify 3 ,
.Xr SSL_get_verify_result 3
.Sh HISTORY
Previous to OpenSSL 0.9.7, the
.Fa arg
argument to
.Fn SSL_CTX_set_cert_verify_callback
was ignored, and
.Fa callback
was called
simply as
.Ft int
.Fn (*callback) "X509_STORE_CTX *" .
To compile software written for previous versions of OpenSSL,
a dummy argument will have to be added to
.Fa callback .
.Sh CAVEATS
Do not mix the verification callback described in this function with the
.Fa verify_callback
function called during the verification process.
The latter is set using the
.Xr SSL_CTX_set_verify 3
family of functions.
.Pp
Providing a complete verification procedure including certificate purpose
settings, etc., is a complex task.
The built-in procedure is quite powerful and in most cases it should be
sufficient to modify its behaviour using the
.Fa verify_callback
function.
.Sh BUGS
.Fn SSL_CTX_set_cert_verify_callback
does not provide diagnostic information.
